(1) Field of the Invention
The present invention relates to heavy duty pneumatic tires having excellent uneven wear resistance.
(2) Related Art Statement
Recently, the radial carcass structure has come to mainly be employed in the case of heavy duty pneumatic tires. When tires of this structure are used in heavy vehicles such as trucks or buses particularly in driving wheels or in driven wheels, so-called uneven wearing often occurs far before the tires do not reach a complete wear life span, thereby causing poor appearance. Further, it is feared that when the tires are continuously used as they are, such a use leads to breakage of land portions called rib punches, thereby causing problems in tire performance.
In order to reduce uneven wear resistance of treads, there have been many proposals on crown contour configurations and tread patterns, particularly on sips arrangements. However, appropriate countermeasures for preventing uneven wearing have not yet been established.
For reference, as typical known literature, for example, there are available U.S. Pat. No. 4,155,392 in which the crown contour configuration is varied, and U.S. Pat. No. 3,550,665 in which sipes are provided on opposite side portions of each of a ribs.
Further, as in the case of U.S. Pat. No. 4,200,134, in order to prevent uneven wearing around grooves, a countermeasure is proposed to prevent stress concentration upon projections of land portions arranged corresponding to a zigzag groove by means of a stress-mitigating rib having the same surface level as that of the land portions of the tread while being spaced from the land portions via the groove. However, in such a countermeasure, the stress-mitigating rib itself tends to peel off. Thus, even if the occurrence of uneven wearing can be retarded, it cannot ultimately be prevented.
The above uneven wearing phenomenon occurring in the tires of this type as a matter of course depends upon running conditions, road surface conditions, etc. When tires are run on recently greatly consolidated highway roads exclusively for automobiles over an extended period of time, the speed of wearing which changes the contour configuration of the tire varies depending upon external forces (forces inputted to the tire) exerted from the tread in the ground-contacting area of the tire. Under the circumstances, wearing is acceleratingly accumulated and promoted in the area at which the wearing is faster.
To the contrary, there are conventional techniques for increasing a ground-contact pressure at a portion at which uneven wearing is more likely to occur or techniques for reducing shearing forces (by notches or the like) so as to control or retard acceleratingly promoted wearing. In such techniques, even if accelerated wearing can be retarded, wearing cannot be prevented from appearing soon. Further, it was often observed that load sharing of the tire varies owing to the wearing, and that uneven wearing occurs.